To us, the main gist of the Trisagion is a Christological prayer (except the end where you say, Glory to the Father, the Son, and the Holy Spirit...), whereas to the EOs, it is a Trinitarian prayer, from beginning to end.
There are indications among early church fathers that a triple holy can be interpreted either way, whether it be to Christ or to the Trinity, both seemed acceptable. Because of the Chalcedonian situation, polemical problems have increased. This phrase was added by OOs as a way to combat Nestorianism, whereas the shock this received by the EOs lead to officially making it a canon that this is a hymn only to the Trinity because the Trinity was not crucified for us.
In the Agpeya, as well as in the Coptic Church liturgically, it is actually three different phrases:
Holy God, Holy Mighty, Holy Immortal, who was....
1. born of the Virgin,
2. crucified for us,
3. rose from the dead and ascended to the heavens,
have mercy on us.
On regular days, we say all three. On the Nativity feast, we repeat number 1 three times. On Passion Week we repeat number 2 three times. From the Resurrection feast to the day before Ascension feast, we only say the first part of number 3 three times, and then from the Ascension feast to the rest of the 50 days we say all of number 3 three times. Finally, on the day of Theophany, we say something different: "baptized in the Jordan," three times.
And...this is chanted in Greek, not Coptic. (fun fact: not everything we chant in the Coptic Church that is not English/Arabic is Coptic, but can be Greek; we've been told we have terrible pronunciations though...I like to think of it as the Coptic accent of Greek